A photodetector receives light as an input signal and produces an electrical voltage or current whose magnitude is proportional to the intensity of the light. A wide variety of imaging and non-imaging photodetectors, operating by various principles, are known. Some photodetectors are sensitive to a wavelength band, while others are sensitive to specific wavelengths. The photodetector may include integral amplification of the output electrical signal, a particularly useful feature where the light intensity of interest is small.
One type of integrated detector and amplifier is an avalanche photodetector. In the avalanche photodetector, a semiconductor absorber produces primary charge carriers (i.e., electrons and/or holes) responsive to the input light signal, and an integral avalanche multiplication region produces a larger number of secondary charge carriers generated by the primary charge carriers.
The effective absorption wavelength range of the semiconductor absorber is a function of the bandgap of its semiconductor material. Currently, there are operable semiconductor materials for some light wavelengths and not for others. For example, the widely used Nd:YAG laser produces light at 1.064 micrometers wavelength. This wavelength falls at or above the long-wavelength limit of infrared-enhanced silicon photodetectors. Due to the indirect bandgap of silicon, silicon photodetectors must have thick active regions in order to reach even modest efficiencies. The thick active region limits the maximum speed of operation of the photodetector. GaInAs material, on the other hand, that is tuned to this wavelength, is not lattice matched to available substrates, resulting in low material quality and poor device performance. GaInAs photodetectors lattice matched to available substrates suffer from excessive thermal currents that degrade performance. Proposed solutions such as the use of ultrathin layers and the use of exotic materials such as GaNAs are complex and expensive. As a result, high-sensitivity, high-speed avalanche photodetectors for light at the 1.064 micrometer wavelength are not practical. There are other wavelengths as well for which avalanche photodetectors are not available.
There is a need for a design approach that allows for the fabrication of an effective avalanche photodetector for specific wavelengths of light, such as 1.064 micrometers wavelength. The approach must provide for an effective light absorber for the selected wavelength, and also for good semiconductor quality to achieve good effectiveness of the conversion of light energy to electrical energy. The present invention fulfills this need, and further provides related advantages.